Membrane-based gas separations are recognized as a solution to global energy needs with advantages in cost and performance. Polymer membranes have been investigated for gas separations for several decades due to low material costs, ease of processing, and available compact geometries. However, the separation performance of polymer membranes has not shown substantial improvements in recent years. Furthermore, polymer materials cannot withstand harsh operating conditions including high temperature and aggressive chemical environments. As an alternative approach, tubular zeolite membranes have been developed for industrial gas separations. As a crystalline structure, tubular zeolite membranes often outperform other separation technologies. However, commercialization efforts have been hampered due to high manufacturing costs, poor reproducibility, and a relatively low surface area/volume ratio.
Carbon molecular sieve membranes (CMSMs) can potentially overcome the limitations of polymer and zeolite membranes for industrial gas separations. CMSMs have shown high thermal and chemical stability and high permselectivity. The high permselectivity is attributed to high porosity and combined connectivity of micropores and ultra micropores below 0.4 nm. Different from zeolite, CMSMs include irregular micropores created by pyrolysis of a polymer precursor where the thermally unstable polymer components are evaporated from a thermally stable backbone polymer. The preparation involves many steps, such as precursor selection and coating, multiple thermal processing, and post-treatment. It is known that the separation performance is strongly dependent on the preparation conditions. However, CMSMs can be tailored to achieve specific separations by optimizing the preparation parameters.
Extensive research has been carried out to understand the correlation between CMSM performance and the preparation parameters. Thermosetting polymers as precursors have been extensively studied, including polyacrylonitrile, polyimide, polyfurfuryl alcohol, phenolic resin, and blending of polymers with different coating techniques, such as spin coating, spray coating, and dip-coating. Pyrolysis conditions, such as gaseous atmosphere, oxidation, and thermal processing conditions are also widely studied. These approaches have resulted in the preparation of selective CMSMs for several challenging gas separations, such as H2/CO2, O2/N2, CO2/CH4, and C3H6/C3H8. Although new understandings and incremental progress has been reported, to date the applications are still limited to laboratory scale.
Further improvements in separation performance and membrane module design are needed for cost-effective industrial scale CMSM systems. Several approaches to the preparation of a high permeance CMSM have been reported by tuning the preparation steps, including polymer processing, atmospheric condition, thermal oxidation, pyrolysis temperature, and post-treatment. While these approaches demonstrate possibilities to realize high permeance, the improvements are still modest. The micropore formation is a natural consequence of structural declinations, and has posed formidable difficulties to simultaneously achieve high permeance and selectivity.